1. Field of Invention
This invention relates techniques for performing packet loss or Frame Erasure Concealment (FEC).
2. Description of Related Art
Frame Erasure Concealment (FEC) algorithms hide transmission losses in a speech communication system where an input speech signal is encoded and packetized at a transmitter, sent over a network (of any sort), and received at a receiver that decodes the packet and plays the speech output. Many of the standard CELP-based speech coders, such as G.723.1, G.728, and G.729, have FEC algorithms built-in or proposed in their standards.
The objective of FEC is to generate a synthetic speech signal to cover missing data in a received bit-stream. Ideally, the synthesized signal will have the same timbre and spectral characteristics as the missing signal, and will not create unnatural artifacts. Since speech signals are often locally stationary, it is possible to use the signals past history to generate a reasonable approximation to the missing segment. If the erasures aren't too long, and the erasure does not land in a region where the signal is rapidly changing, the erasures may be inaudible after concealment.
Prior systems did employ pitch waveform replication techniques to conceal frame erasures, such as, for example, D. J. Goodman et al., Waveform Substitution Techniques for Recovering Missing Speech Segments in Packet Voice Communications, Vol. 34, No. 6 IEEE Trans. on Acoustics, Speech, and Signal Processing 1440-48 (December 1996) and O. J. Wasem et al., The Effect of Waveform Substitution on the Quality of PCM Packet Communications, Vol. 36, No 3 IEEE Transactions on Acoustics, Speech, and Signal Processing 342-48 (March 1988).
Although pitch waveform replication and overlap-add techniques have been used to synthesize signals to conceal lost frames of speech data, these techniques sometimes result in unnatural artifacts that are unsatisfactory to the listener.